(* Title: HOL/ex/BigO.thy
Authors: Jeremy Avigad and Kevin Donnelly; proofs tidied by LCP
*)
section \<open>Big O notation\<close>
theory BigO
imports
Complex_Main
"HOL-Library.Function_Algebras"
"HOL-Library.Set_Algebras"
begin
text \<open>
This library is designed to support asymptotic ``big O'' calculations,
i.e.~reasoning with expressions of the form \<open>f = O(g)\<close> and \<open>f = g + O(h)\<close>.
An earlier version of this library is described in detail in \<^cite>\<open>"Avigad-Donnelly"\<close>.
The main changes in this version are as follows:
\<^item> We have eliminated the \<open>O\<close> operator on sets. (Most uses of this seem
to be inessential.)
\<^item> We no longer use \<open>+\<close> as output syntax for \<open>+o\<close>
\<^item> Lemmas involving \<open>sumr\<close> have been replaced by more general lemmas
involving `\<open>sum\<close>.
\<^item> The library has been expanded, with e.g.~support for expressions of
the form \<open>f < g + O(h)\<close>.
Note also since the Big O library includes rules that demonstrate set
inclusion, to use the automated reasoners effectively with the library one
should redeclare the theorem \<open>subsetI\<close> as an intro rule, rather than as an
\<open>intro!\<close> rule, for example, using \<^theory_text>\<open>declare subsetI [del, intro]\<close>.
\<close>
subsection \<open>Definitions\<close>
definition bigo :: "('a \<Rightarrow> 'b::linordered_idom) \<Rightarrow> ('a \<Rightarrow> 'b) set" ("(1O'(_'))")
where "O(f:: 'a \<Rightarrow> 'b) = {h. \<exists>c. \<forall>x. \<bar>h x\<bar> \<le> c * \<bar>f x\<bar>}"
lemma bigo_pos_const:
"(\<exists>c::'a::linordered_idom. \<forall>x. \<bar>h x\<bar> \<le> c * \<bar>f x\<bar>) \<longleftrightarrow>
(\<exists>c. 0 < c \<and> (\<forall>x. \<bar>h x\<bar> \<le> c * \<bar>f x\<bar>))"
by (metis (no_types, opaque_lifting) abs_ge_zero abs_not_less_zero abs_of_nonneg dual_order.trans
mult_1 zero_less_abs_iff zero_less_mult_iff zero_less_one)
lemma bigo_alt_def: "O(f) = {h. \<exists>c. 0 < c \<and> (\<forall>x. \<bar>h x\<bar> \<le> c * \<bar>f x\<bar>)}"
by (auto simp add: bigo_def bigo_pos_const)
lemma bigo_elt_subset [intro]: "f \<in> O(g) \<Longrightarrow> O(f) \<le> O(g)"
apply (auto simp add: bigo_alt_def)
by (metis (no_types, opaque_lifting) mult.assoc mult_le_cancel_iff2 order.trans
zero_less_mult_iff)
lemma bigo_refl [intro]: "f \<in> O(f)"
using bigo_def comm_monoid_mult_class.mult_1 dual_order.eq_iff by blast
lemma bigo_zero: "0 \<in> O(g)"
using bigo_def mult_le_cancel_left1 by fastforce
lemma bigo_zero2: "O(\<lambda>x. 0) = {\<lambda>x. 0}"
by (auto simp add: bigo_def)
lemma bigo_plus_self_subset [intro]: "O(f) + O(f) \<subseteq> O(f)"
apply (auto simp add: bigo_alt_def set_plus_def)
apply (rule_tac x = "c + ca" in exI)
by (smt (verit, best) abs_triangle_ineq add_mono add_pos_pos comm_semiring_class.distrib dual_order.trans)
lemma bigo_plus_idemp [simp]: "O(f) + O(f) = O(f)"
by (simp add: antisym bigo_plus_self_subset bigo_zero set_zero_plus2)
lemma bigo_plus_subset [intro]: "O(f + g) \<subseteq> O(f) + O(g)"
apply (rule subsetI)
apply (auto simp add: bigo_def bigo_pos_const func_plus set_plus_def)
apply (subst bigo_pos_const [symmetric])+
apply (rule_tac x = "\<lambda>n. if \<bar>g n\<bar> \<le> \<bar>f n\<bar> then x n else 0" in exI)
apply (rule conjI)
apply (rule_tac x = "c + c" in exI)
apply (clarsimp)
apply (smt (verit, ccfv_threshold) mult.commute abs_triangle_ineq add_le_cancel_left dual_order.trans mult.left_commute mult_2 mult_le_cancel_iff2)
apply (simp add: order_less_le)
apply (rule_tac x = "\<lambda>n. if \<bar>f n\<bar> < \<bar>g n\<bar> then x n else 0" in exI)
apply (rule conjI)
apply (rule_tac x = "c + c" in exI)
apply auto
apply (subgoal_tac "c * \<bar>f xa + g xa\<bar> \<le> (c + c) * \<bar>g xa\<bar>")
apply (metis mult_2 order.trans)
apply simp
done
lemma bigo_plus_subset2 [intro]: "A \<subseteq> O(f) \<Longrightarrow> B \<subseteq> O(f) \<Longrightarrow> A + B \<subseteq> O(f)"
using bigo_plus_idemp set_plus_mono2 by blast
lemma bigo_plus_eq: "\<forall>x. 0 \<le> f x \<Longrightarrow> \<forall>x. 0 \<le> g x \<Longrightarrow> O(f + g) = O(f) + O(g)"
apply (rule equalityI)
apply (rule bigo_plus_subset)
apply (simp add: bigo_alt_def set_plus_def func_plus)
apply clarify
apply (rule_tac x = "max c ca" in exI)
by (smt (verit, del_insts) add.commute abs_triangle_ineq add_mono_thms_linordered_field(3) distrib_left less_max_iff_disj linorder_not_less max.orderE max_mult_distrib_right order_le_less)
lemma bigo_bounded_alt: "\<forall>x. 0 \<le> f x \<Longrightarrow> \<forall>x. f x \<le> c * g x \<Longrightarrow> f \<in> O(g)"
by (simp add: bigo_def) (metis abs_mult abs_of_nonneg order_trans)
lemma bigo_bounded: "\<forall>x. 0 \<le> f x \<Longrightarrow> \<forall>x. f x \<le> g x \<Longrightarrow> f \<in> O(g)"
by (metis bigo_bounded_alt mult_1)
lemma bigo_bounded2: "\<forall>x. lb x \<le> f x \<Longrightarrow> \<forall>x. f x \<le> lb x + g x \<Longrightarrow> f \<in> lb +o O(g)"
by (simp add: add.commute bigo_bounded diff_le_eq set_minus_imp_plus)
lemma bigo_abs: "(\<lambda>x. \<bar>f x\<bar>) =o O(f)"
by (smt (verit, del_insts) abs_abs bigo_def bigo_refl mem_Collect_eq)
lemma bigo_abs2: "f =o O(\<lambda>x. \<bar>f x\<bar>)"
by (smt (verit, del_insts) abs_abs bigo_def bigo_refl mem_Collect_eq)
lemma bigo_abs3: "O(f) = O(\<lambda>x. \<bar>f x\<bar>)"
using bigo_abs bigo_abs2 bigo_elt_subset by blast
lemma bigo_abs4: assumes "f =o g +o O(h)" shows "(\<lambda>x. \<bar>f x\<bar>) =o (\<lambda>x. \<bar>g x\<bar>) +o O(h)"
proof -
{ assume *: "f - g \<in> O(h)"
have "(\<lambda>x. \<bar>f x\<bar> - \<bar>g x\<bar>) =o O(\<lambda>x. \<bar>\<bar>f x\<bar> - \<bar>g x\<bar>\<bar>)"
by (rule bigo_abs2)
also have "\<dots> \<subseteq> O(\<lambda>x. \<bar>f x - g x\<bar>)"
by (simp add: abs_triangle_ineq3 bigo_bounded bigo_elt_subset)
also have "\<dots> \<subseteq> O(f - g)"
using bigo_abs3 by fastforce
also from * have "\<dots> \<subseteq> O(h)"
by (rule bigo_elt_subset)
finally have "(\<lambda>x. \<bar>f x\<bar> - \<bar>g x\<bar>) \<in> O(h)" . }
then show ?thesis
by (smt (verit) assms bigo_alt_def fun_diff_def mem_Collect_eq set_minus_imp_plus set_plus_imp_minus)
qed
lemma bigo_abs5: "f =o O(g) \<Longrightarrow> (\<lambda>x. \<bar>f x\<bar>) =o O(g)"
by (auto simp: bigo_def)
lemma bigo_elt_subset2 [intro]:
assumes *: "f \<in> g +o O(h)"
shows "O(f) \<subseteq> O(g) + O(h)"
proof -
note *
also have "g +o O(h) \<subseteq> O(g) + O(h)"
by (auto del: subsetI)
also have "\<dots> = O(\<lambda>x. \<bar>g x\<bar>) + O(\<lambda>x. \<bar>h x\<bar>)"
by (subst bigo_abs3 [symmetric])+ (rule refl)
also have "\<dots> = O((\<lambda>x. \<bar>g x\<bar>) + (\<lambda>x. \<bar>h x\<bar>))"
by (rule bigo_plus_eq [symmetric]) auto
finally have "f \<in> \<dots>" .
then have "O(f) \<subseteq> \<dots>"
by (elim bigo_elt_subset)
also have "\<dots> = O(\<lambda>x. \<bar>g x\<bar>) + O(\<lambda>x. \<bar>h x\<bar>)"
by (rule bigo_plus_eq, auto)
finally show ?thesis
by (simp flip: bigo_abs3)
qed
lemma bigo_mult [intro]: "O(f)*O(g) \<subseteq> O(f * g)"
apply (rule subsetI)
apply (subst bigo_def)
apply (clarsimp simp add: bigo_alt_def set_times_def func_times)
apply (rule_tac x = "c * ca" in exI)
by (smt (verit, ccfv_threshold) mult.commute mult.assoc abs_ge_zero abs_mult dual_order.trans mult_mono)
lemma bigo_mult2 [intro]: "f *o O(g) \<subseteq> O(f * g)"
by (metis bigo_mult bigo_refl dual_order.trans mult.commute set_times_mono4)
lemma bigo_mult3: "f \<in> O(h) \<Longrightarrow> g \<in> O(j) \<Longrightarrow> f * g \<in> O(h * j)"
using bigo_mult mult.commute mult.commute set_times_intro subsetD by blast
lemma bigo_mult4 [intro]: "f \<in> k +o O(h) \<Longrightarrow> g * f \<in> (g * k) +o O(g * h)"
by (metis bigo_mult3 bigo_refl left_diff_distrib' mult.commute set_minus_imp_plus set_plus_imp_minus)
lemma bigo_mult5:
fixes f :: "'a \<Rightarrow> 'b::linordered_field"
assumes "\<forall>x. f x \<noteq> 0"
shows "O(f * g) \<subseteq> f *o O(g)"
proof
fix h
assume "h \<in> O(f * g)"
then have "(\<lambda>x. 1 / (f x)) * h \<in> (\<lambda>x. 1 / f x) *o O(f * g)"
by auto
also have "\<dots> \<subseteq> O((\<lambda>x. 1 / f x) * (f * g))"
by (rule bigo_mult2)
also have "(\<lambda>x. 1 / f x) * (f * g) = g"
using assms by auto
finally have "(\<lambda>x. (1::'b) / f x) * h \<in> O(g)" .
then have "f * ((\<lambda>x. (1::'b) / f x) * h) \<in> f *o O(g)"
by auto
also have "f * ((\<lambda>x. (1::'b) / f x) * h) = h"
by (simp add: assms times_fun_def)
finally show "h \<in> f *o O(g)" .
qed
lemma bigo_mult6: "\<forall>x. f x \<noteq> 0 \<Longrightarrow> O(f * g) = f *o O(g)"
for f :: "'a \<Rightarrow> 'b::linordered_field"
by (simp add: bigo_mult2 bigo_mult5 subset_antisym)
lemma bigo_mult7: "\<forall>x. f x \<noteq> 0 \<Longrightarrow> O(f * g) \<subseteq> O(f) * O(g)"
for f :: "'a \<Rightarrow> 'b::linordered_field"
by (metis bigo_mult6 bigo_refl mult.commute set_times_mono4)
lemma bigo_mult8: "\<forall>x. f x \<noteq> 0 \<Longrightarrow> O(f * g) = O(f) * O(g)"
for f :: "'a \<Rightarrow> 'b::linordered_field"
by (simp add: bigo_mult bigo_mult7 subset_antisym)
lemma bigo_minus [intro]: "f \<in> O(g) \<Longrightarrow> - f \<in> O(g)"
by (auto simp add: bigo_def fun_Compl_def)
lemma bigo_minus2:
assumes "f \<in> g +o O(h)" shows "- f \<in> -g +o O(h)"
proof -
have "- f + g \<in> O(h)"
by (metis assms bigo_minus minus_diff_eq set_plus_imp_minus uminus_add_conv_diff)
then show ?thesis
by (simp add: set_minus_imp_plus)
qed
lemma bigo_minus3: "O(- f) = O(f)"
by (auto simp add: bigo_def fun_Compl_def)
lemma bigo_plus_absorb_lemma1:
assumes *: "f \<in> O(g)"
shows "f +o O(g) \<subseteq> O(g)"
using assms bigo_plus_idemp set_plus_mono4 by blast
lemma bigo_plus_absorb_lemma2:
assumes *: "f \<in> O(g)"
shows "O(g) \<subseteq> f +o O(g)"
proof -
from * have "- f \<in> O(g)"
by auto
then have "- f +o O(g) \<subseteq> O(g)"
by (elim bigo_plus_absorb_lemma1)
then have "f +o (- f +o O(g)) \<subseteq> f +o O(g)"
by auto
also have "f +o (- f +o O(g)) = O(g)"
by (simp add: set_plus_rearranges)
finally show ?thesis .
qed
lemma bigo_plus_absorb [simp]: "f \<in> O(g) \<Longrightarrow> f +o O(g) = O(g)"
by (simp add: bigo_plus_absorb_lemma1 bigo_plus_absorb_lemma2 subset_antisym)
lemma bigo_plus_absorb2 [intro]: "f \<in> O(g) \<Longrightarrow> A \<subseteq> O(g) \<Longrightarrow> f +o A \<subseteq> O(g)"
using bigo_plus_absorb set_plus_mono by blast
lemma bigo_add_commute_imp: "f \<in> g +o O(h) \<Longrightarrow> g \<in> f +o O(h)"
by (metis bigo_minus minus_diff_eq set_minus_imp_plus set_plus_imp_minus)
lemma bigo_add_commute: "f \<in> g +o O(h) \<longleftrightarrow> g \<in> f +o O(h)"
using bigo_add_commute_imp by blast
lemma bigo_const1: "(\<lambda>x. c) \<in> O(\<lambda>x. 1)"
by (auto simp add: bigo_def ac_simps)
lemma bigo_const2 [intro]: "O(\<lambda>x. c) \<subseteq> O(\<lambda>x. 1)"
by (metis bigo_elt_subset bigo_const1)
lemma bigo_const3: "c \<noteq> 0 \<Longrightarrow> (\<lambda>x. 1) \<in> O(\<lambda>x. c)"
for c :: "'a::linordered_field"
by (metis bigo_bounded_alt le_numeral_extra(4) nonzero_divide_eq_eq zero_less_one_class.zero_le_one)
lemma bigo_const4: "c \<noteq> 0 \<Longrightarrow> O(\<lambda>x. 1) \<subseteq> O(\<lambda>x. c)"
for c :: "'a::linordered_field"
by (metis bigo_elt_subset bigo_const3)
lemma bigo_const [simp]: "c \<noteq> 0 \<Longrightarrow> O(\<lambda>x. c) = O(\<lambda>x. 1)"
for c :: "'a::linordered_field"
by (metis equalityI bigo_const2 bigo_const4)
lemma bigo_const_mult1: "(\<lambda>x. c * f x) \<in> O(f)"
by (simp add: bigo_def) (metis abs_mult dual_order.refl)
lemma bigo_const_mult2: "O(\<lambda>x. c * f x) \<subseteq> O(f)"
by (metis bigo_elt_subset bigo_const_mult1)
lemma bigo_const_mult3: "c \<noteq> 0 \<Longrightarrow> f \<in> O(\<lambda>x. c * f x)"
for c :: "'a::linordered_field"
by (simp add: bigo_def) (metis abs_mult field_class.field_divide_inverse mult.commute nonzero_divide_eq_eq order_refl)
lemma bigo_const_mult4: "c \<noteq> 0 \<Longrightarrow> O(f) \<subseteq> O(\<lambda>x. c * f x)"
for c :: "'a::linordered_field"
by (simp add: bigo_const_mult3 bigo_elt_subset)
lemma bigo_const_mult [simp]: "c \<noteq> 0 \<Longrightarrow> O(\<lambda>x. c * f x) = O(f)"
for c :: "'a::linordered_field"
by (simp add: bigo_const_mult2 bigo_const_mult4 subset_antisym)
lemma bigo_const_mult5 [simp]: "(\<lambda>x. c) *o O(f) = O(f)" if "c \<noteq> 0"
for c :: "'a::linordered_field"
proof
show "O(f) \<subseteq> (\<lambda>x. c) *o O(f)"
using that
apply (clarsimp simp add: bigo_def elt_set_times_def func_times)
apply (rule_tac x = "\<lambda>y. inverse c * x y" in exI)
apply (simp add: mult.assoc [symmetric] abs_mult)
apply (rule_tac x = "\<bar>inverse c\<bar> * ca" in exI)
apply auto
done
have "O(\<lambda>x. c * f x) \<subseteq> O(f)"
by (simp add: bigo_const_mult2)
then show "(\<lambda>x. c) *o O(f) \<subseteq> O(f)"
using order_trans[OF bigo_mult2] by (force simp add: times_fun_def)
qed
lemma bigo_const_mult6 [intro]: "(\<lambda>x. c) *o O(f) \<subseteq> O(f)"
apply (auto intro!: simp add: bigo_def elt_set_times_def func_times)
apply (rule_tac x = "ca * \<bar>c\<bar>" in exI)
by (smt (verit, ccfv_SIG) ab_semigroup_mult_class.mult_ac(1) abs_abs abs_le_self_iff abs_mult le_cases3 mult.commute mult_left_mono)
lemma bigo_const_mult7 [intro]:
assumes *: "f =o O(g)"
shows "(\<lambda>x. c * f x) =o O(g)"
proof -
from * have "(\<lambda>x. c) * f =o (\<lambda>x. c) *o O(g)"
by auto
also have "(\<lambda>x. c) * f = (\<lambda>x. c * f x)"
by (simp add: func_times)
also have "(\<lambda>x. c) *o O(g) \<subseteq> O(g)"
by (auto del: subsetI)
finally show ?thesis .
qed
lemma bigo_compose1: "f =o O(g) \<Longrightarrow> (\<lambda>x. f (k x)) =o O(\<lambda>x. g (k x))"
by (auto simp: bigo_def)
lemma bigo_compose2: "f =o g +o O(h) \<Longrightarrow> (\<lambda>x. f (k x)) =o (\<lambda>x. g (k x)) +o O(\<lambda>x. h(k x))"
by (smt (verit, best) set_minus_plus bigo_def fun_diff_def mem_Collect_eq)
subsection \<open>Sum\<close>
lemma bigo_sum_main:
assumes "\<forall>x. \<forall>y \<in> A x. 0 \<le> h x y" and "\<forall>x. \<forall>y \<in> A x. \<bar>f x y\<bar> \<le> c * h x y"
shows "(\<lambda>x. \<Sum>y \<in> A x. f x y) =o O(\<lambda>x. \<Sum>y \<in> A x. h x y)"
proof -
have "(\<Sum>i\<in>A x. \<bar>f x i\<bar>) \<le> \<bar>c\<bar> * sum (h x) (A x)" for x
by (smt (verit, ccfv_threshold) assms abs_mult_pos abs_of_nonneg abs_of_nonpos dual_order.trans le_cases3 neg_0_le_iff_le sum_distrib_left sum_mono)
then show ?thesis
using assms by (fastforce simp add: bigo_def sum_nonneg intro: order_trans [OF sum_abs])
qed
lemma bigo_sum1: "\<forall>x y. 0 \<le> h x y \<Longrightarrow>
\<exists>c. \<forall>x y. \<bar>f x y\<bar> \<le> c * h x y \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. f x y) =o O(\<lambda>x. \<Sum>y \<in> A x. h x y)"
by (metis (no_types) bigo_sum_main)
lemma bigo_sum2: "\<forall>y. 0 \<le> h y \<Longrightarrow>
\<exists>c. \<forall>y. \<bar>f y\<bar> \<le> c * (h y) \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. f y) =o O(\<lambda>x. \<Sum>y \<in> A x. h y)"
by (rule bigo_sum1) auto
lemma bigo_sum3: "f =o O(h) \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. l x y * f (k x y)) =o O(\<lambda>x. \<Sum>y \<in> A x. \<bar>l x y * h (k x y)\<bar>)"
apply (rule bigo_sum1)
using abs_ge_zero apply blast
apply (clarsimp simp: bigo_def)
by (smt (verit, ccfv_threshold) abs_mult abs_not_less_zero mult.left_commute mult_le_cancel_left)
lemma bigo_sum4: "f =o g +o O(h) \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. l x y * f (k x y)) =o
(\<lambda>x. \<Sum>y \<in> A x. l x y * g (k x y)) +o
O(\<lambda>x. \<Sum>y \<in> A x. \<bar>l x y * h (k x y)\<bar>)"
using bigo_sum3 [of "f-g" h l k A]
apply (simp add: algebra_simps sum_subtractf)
by (smt (verit) bigo_alt_def minus_apply set_minus_imp_plus set_plus_imp_minus mem_Collect_eq)
lemma bigo_sum5: "f =o O(h) \<Longrightarrow> \<forall>x y. 0 \<le> l x y \<Longrightarrow>
\<forall>x. 0 \<le> h x \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. l x y * f (k x y)) =o
O(\<lambda>x. \<Sum>y \<in> A x. l x y * h (k x y))"
using bigo_sum3 [of f h l k A] by simp
lemma bigo_sum6: "f =o g +o O(h) \<Longrightarrow> \<forall>x y. 0 \<le> l x y \<Longrightarrow>
\<forall>x. 0 \<le> h x \<Longrightarrow>
(\<lambda>x. \<Sum>y \<in> A x. l x y * f (k x y)) =o
(\<lambda>x. \<Sum>y \<in> A x. l x y * g (k x y)) +o
O(\<lambda>x. \<Sum>y \<in> A x. l x y * h (k x y))"
using bigo_sum5 [of "f-g" h l k A]
apply (simp add: algebra_simps sum_subtractf)
by (smt (verit, del_insts) bigo_alt_def set_minus_imp_plus minus_apply set_plus_imp_minus mem_Collect_eq)
subsection \<open>Misc useful stuff\<close>
lemma bigo_useful_add: "f =o O(h) \<Longrightarrow> g =o O(h) \<Longrightarrow> f + g =o O(h)"
using bigo_plus_idemp set_plus_intro by blast
lemma bigo_useful_const_mult: "c \<noteq> 0 \<Longrightarrow> (\<lambda>x. c) * f =o O(h) \<Longrightarrow> f =o O(h)"
for c :: "'a::linordered_field"
using bigo_elt_subset bigo_mult6 by fastforce
lemma bigo_fix: "(\<lambda>x::nat. f (x + 1)) =o O(\<lambda>x. h (x + 1)) \<Longrightarrow> f 0 = 0 \<Longrightarrow> f =o O(h)"
by (simp add: bigo_alt_def) (metis abs_eq_0_iff abs_ge_zero abs_mult abs_of_pos not0_implies_Suc)
lemma bigo_fix2:
"(\<lambda>x. f ((x::nat) + 1)) =o (\<lambda>x. g(x + 1)) +o O(\<lambda>x. h(x + 1)) \<Longrightarrow>
f 0 = g 0 \<Longrightarrow> f =o g +o O(h)"
apply (rule set_minus_imp_plus [OF bigo_fix])
apply (smt (verit, del_insts) bigo_alt_def fun_diff_def set_plus_imp_minus mem_Collect_eq)
apply simp
done
subsection \<open>Less than or equal to\<close>
definition lesso :: "('a \<Rightarrow> 'b::linordered_idom) \<Rightarrow> ('a \<Rightarrow> 'b) \<Rightarrow> 'a \<Rightarrow> 'b" (infixl "<o" 70)
where "f <o g = (\<lambda>x. max (f x - g x) 0)"
lemma bigo_lesseq1: "f =o O(h) \<Longrightarrow> \<forall>x. \<bar>g x\<bar> \<le> \<bar>f x\<bar> \<Longrightarrow> g =o O(h)"
by (smt (verit, del_insts) bigo_def mem_Collect_eq order_trans)
lemma bigo_lesseq2: "f =o O(h) \<Longrightarrow> \<forall>x. \<bar>g x\<bar> \<le> f x \<Longrightarrow> g =o O(h)"
by (metis (mono_tags, lifting) abs_ge_zero abs_of_nonneg bigo_lesseq1 dual_order.trans)
lemma bigo_lesseq3: "f =o O(h) \<Longrightarrow> \<forall>x. 0 \<le> g x \<Longrightarrow> \<forall>x. g x \<le> f x \<Longrightarrow> g =o O(h)"
by (meson bigo_bounded bigo_elt_subset subsetD)
lemma bigo_lesseq4: "f =o O(h) \<Longrightarrow> \<forall>x. 0 \<le> g x \<Longrightarrow> \<forall>x. g x \<le> \<bar>f x\<bar> \<Longrightarrow> g =o O(h)"
by (metis abs_of_nonneg bigo_lesseq1)
lemma bigo_lesso1: "\<forall>x. f x \<le> g x \<Longrightarrow> f <o g =o O(h)"
by (smt (verit, del_insts) abs_ge_zero add_0 bigo_abs3 bigo_bounded diff_le_eq lesso_def max_def order_refl)
lemma bigo_lesso2: "f =o g +o O(h) \<Longrightarrow> \<forall>x. 0 \<le> k x \<Longrightarrow> \<forall>x. k x \<le> f x \<Longrightarrow> k <o g =o O(h)"
unfolding lesso_def
apply (rule bigo_lesseq4 [of "f-g"])
apply (erule set_plus_imp_minus)
using max.cobounded2 apply blast
by (smt (verit) abs_ge_zero abs_of_nonneg diff_ge_0_iff_ge diff_mono diff_self fun_diff_def order_refl max.coboundedI2 max_def)
lemma bigo_lesso3: "f =o g +o O(h) \<Longrightarrow> \<forall>x. 0 \<le> k x \<Longrightarrow> \<forall>x. g x \<le> k x \<Longrightarrow> f <o k =o O(h)"
unfolding lesso_def
apply (rule bigo_lesseq4 [of "f-g"])
apply (erule set_plus_imp_minus)
using max.cobounded2 apply blast
by (smt (verit) abs_eq_iff abs_ge_zero abs_if abs_minus_le_zero diff_left_mono fun_diff_def le_max_iff_disj order.trans order_eq_refl)
lemma bigo_lesso4:
fixes k :: "'a \<Rightarrow> 'b::linordered_field"
assumes f: "f <o g =o O(k)" and g: "g =o h +o O(k)"
shows "f <o h =o O(k)"
proof -
have "g - h \<in> O(k)"
by (simp add: g set_plus_imp_minus)
then have "(\<lambda>x. \<bar>g x - h x\<bar>) \<in> O(k)"
using bigo_abs5 by force
then have \<section>: "(\<lambda>x. max (f x - g x) 0) + (\<lambda>x. \<bar>g x - h x\<bar>) \<in> O(k)"
by (metis (mono_tags, lifting) bigo_lesseq1 bigo_useful_add dual_order.eq_iff f lesso_def)
have "\<bar>max (f x - h x) 0\<bar> \<le> ((\<lambda>x. max (f x - g x) 0) + (\<lambda>x. \<bar>g x - h x\<bar>)) x" for x
by (auto simp add: func_plus fun_diff_def algebra_simps split: split_max abs_split)
then show ?thesis
by (smt (verit, ccfv_SIG) \<section> bigo_lesseq2 lesso_def)
qed
lemma bigo_lesso5:
assumes "f <o g =o O(h)" shows "\<exists>C. \<forall>x. f x \<le> g x + C * \<bar>h x\<bar>"
proof -
obtain c where "0 < c" and c: "\<And>x. f x - g x \<le> c * \<bar>h x\<bar>"
using assms by (auto simp: lesso_def bigo_alt_def)
have "\<bar>max (f x - g x) 0\<bar> = max (f x - g x) 0" for x
by (auto simp add: algebra_simps)
then show ?thesis
by (metis c add.commute diff_le_eq)
qed
lemma lesso_add: "f <o g =o O(h) \<Longrightarrow> k <o l =o O(h) \<Longrightarrow> (f + k) <o (g + l) =o O(h)"
unfolding lesso_def
using bigo_useful_add by (fastforce split: split_max intro: bigo_lesseq3)
lemma bigo_LIMSEQ1: "f \<longlonglongrightarrow> 0" if f: "f =o O(g)" and g: "g \<longlonglongrightarrow> 0"
for f g :: "nat \<Rightarrow> real"
proof -
{ fix r::real
assume "0 < r"
obtain c::real where "0 < c" and rc: "\<And>x. \<bar>f x\<bar> \<le> c * \<bar>g x\<bar>"
using f by (auto simp: LIMSEQ_iff bigo_alt_def)
with g \<open>0 < r\<close> obtain no where "\<forall>n\<ge>no. \<bar>g n\<bar> < r/c"
by (fastforce simp: LIMSEQ_iff)
then have "\<exists>no. \<forall>n\<ge>no. \<bar>f n\<bar> < r"
by (metis \<open>0 < c\<close> mult.commute order_le_less_trans pos_less_divide_eq rc) }
then show ?thesis
by (auto simp: LIMSEQ_iff)
qed
lemma bigo_LIMSEQ2:
fixes f g :: "nat \<Rightarrow> real"
assumes "f =o g +o O(h)" "h \<longlonglongrightarrow> 0" and f: "f \<longlonglongrightarrow> a"
shows "g \<longlonglongrightarrow> a"
proof -
have "f - g \<longlonglongrightarrow> 0"
using assms bigo_LIMSEQ1 set_plus_imp_minus by blast
then have "(\<lambda>n. f n - g n) \<longlonglongrightarrow> 0"
by (simp add: fun_diff_def)
then show ?thesis
using Lim_transform_eq f by blast
qed
end